When a Northern Line train passes a red light, a trip cock is triggered and activates three switches.
The rst two operate emergency brakes while the last switch prevents the train from driving at full speed for three minutes from the time the train comes to a stop.
During a routine check on 6 September, the third switch failed to activate when the train approached a stop signal at low speed.
Six of these switches - located under seats behind the driving cars on every Northern and Jubilee Line train - have been replaced in the last two weeks.
The new switches now operate at 6.5 bar pressure, 2.7 bar more than before to ensure they trigger the goslow safety mechanism.